Appropriate Uses for Cob
Michael G. Smith has a background in environmental engineering, ecology, and sustainable resource management. In 1993, along with Ianto Evans and Linda Smiley, he started the Cob Cottage Company, a research and teaching group focused on reviving and improving traditional forms of earthen construction. He is the author of The Cobber's Companion: How to Build Your Own Earthen Home (Cob Cottage Co., 1998) and co-author of The Art of Natural Building: Design, Construction, Resources (New Society, 2002) and The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002). He teaches practical workshops and provides consultation to owner-builders on a wide variety of natural building techniques, site selection, and design. He lives in an intentional community in Northern California. His informative website is www.strawclaywood.com.
Q: What are the weaknesses of cob?
A: Do you mean the structural weaknesses, or the general weaknesses of the system? If you mean structurally, then cob (like all masonry materials) is much stronger in compression than in tension. That means that although they can withstand a great deal of pressure from gravity, cob walls can fail when subjected to large shear forces, such as earthquakes.
If you mean generalized weaknesses, here are a few. Cob, like other earthen building systems (as well as straw and wood) can be damaged by excessive moisture over time. It is important to keep the walls of the building from becoming saturated with water. In cool or moist conditions, cob dries slowly, which can make construction of a cob building very slow under those conditions.
In my opinion, the specifics of the building project (including for example climate, seismic conditions, building size and usage, the local availability of different materials, ease of access to site, social and political factors, to name a few) and the desires and skills of the builders and/or clients determine which building system or systems are most appropriate. There are many specific circumstances under which I would not recommend using cob because another material or system would be more appropriate.
Q: I'm new to this cobbing and I am not sure about how to see through an idea I have for my school garden. My garden consists of 8 raised-beds and we would now like to include a seating area in the center of the garden for reading and observations. Is there a way to make it using straw bales for the form and then covering it with clay? What do you mix with the clay? Will it withstand the cold winters of Wisconsin? Is it expensive? Can I use clay from a farmers field? Do you know of any awesome resources?
A: There are many ways to make a bench using natural materials such as clay and straw. One way would be to make a mixture of clay, sand, and straw, called "cob", and sculpt it into a bench of whatever size and shape you like. Another would be to use straw bales and plaster over them with wither, a clay-based or a lime-based mixture. If the bench is not covered with a roof, a cob bench will probably last quite a bit longer than a straw bale one, as the bales will rot out if they get too wet. For either cob or bales, moisture is a much bigger concern than cold. If you want the bench to last more than a few years, you should put a roof over it and/or cover it with a weather-resisting finish such as a lime-sand plaster. A good book to read for further details about mix ratios, building techniques, etc. would be The Hand-Sculpted House by Evans, Smith, and Smiley (available above).
Q: I and my family will be moving to Mozambique in southern Africa in December. I am really interested in building a home with cob. However, I have some questions over tropical climates and cob. The climate, broadly speaking, is a four month wet season which is the summer and the rest of the year is the dry season. I have heard that a high mass structure such as cob will not do well in a tropical climate. What do you think?
A: The issue is not so much whether the cob will survive the climate (cob does well in very wet regions, if adequately protected), but whether it will make the most comfortable and efficient house in a given climate. In hot, moist tropical zones, temperatures tend to stay high at night. Massive materials such as cob lose their cooling properties under those conditions since the "thermal battery" doesn't get discharged at night and the mass stays warm all the time. In moist tropical climates like those, the usual strategy is to make a lightweight, low mass building high off the ground and with as much ventilation as possible. I don't know much about the climate of Mozambique, but it sounds like a desert with monsoons, similar to parts of the American Southwest. If that's correct, then cob should make a good choice. In those climates the air humidity is basically low all the time except when it's actually raining, so the air cools off at night and heat stored in thermal mass can be discharged. There's one easy way to find out whether cob would be suitable where you are moving. Look at the vernacular architecture in the region. Cultures that have existed in a certain locale for a long time generally find the most efficient solutions given the materials they have available. If massive earth walls are typical, then they probably work well. If not, then there is probably a good reason.
Q: How is the dew point condensate managed within a cob wall? In cold climates this would surely be an issue... but long lived cob in such places is a testament to its irrelevance when done correctly. Can you explain this? How is it possible to avoid the vapor barrier...?
A: No doubt a dew point does occur within cob walls, especially in cold climates, as you suggest. Yet as you also note, cob walls have held up fine for many centuries, even in Northern Europe. I believe this attests to the large capacity of earthen materials to absorb, distribute, and release moisture over time without ill effects. Experience suggests that cob can absorb a great deal of moisture, both from weather and atmosphere and from isolated accidents such as plumbing failures. As long as the cob is allowed to dry out again, there is rarely a problem. When the saturation point is reached, there can be a sudden and spectacular collapse, but that seems to happen only when either the cob is actually sitting in water for some period of time or coated with a non-breathable covering such as cement stucco. So preventative measures are fairly straight-forward: good drainage to prevent flooding, and a breathable surface coating. As well as, of course, an adequate roof. In places where you have a lot of wind-driven rain, you may want to protect the exterior with a weather-resistant but breathable plaster, such as lime. But that is really more to avoid cosmetic damage and slow erosion than because of moisture concerns.
Your question points to one of the big differences between cob and straw bale. Straw, as a biological material, is very susceptible to decomposition by microorganisms, given the right conditions, which include moisture. Once the moisture level gets to a certain point, the straw will begin to rot. Cob is made mainly of geological materials - sand and clay - which are not subject to digestion by microorganisms. The clay tends to preserve the straw in cob because it is more hydrophilic, and thus draws the moisture away. I have seen samples of cob 200 years old, in rainy New Zealand, where the straw inside was still yellow.
Q: Is it possible to combine Rastra block walls (for a basic rectangular house) and do cob detailing on the outside? I want to build an off-grid passive solar home that will last 500 years... but I want it to look great, too, with the neat details you can do with cob. Is this do-able, and if so, any suggestions (like surrounding the panels with chicken wire like some do with bales).
A: An interesting idea. It is possible to get cob or other earthen materials to adhere to almost anything. If you only want to build up a thin layer of earthen plaster, all you need is a surface that is sufficiently rough. I'm not sufficiently familiar with Rastra to know whether it meets the requirements. Certainly you can apply an earthen plaster over a standard brick wall. If the surface is too smooth, you will need to create additional "tooth," either by scratching or etching the surface or by painting on an 'adhesion coat,' made, for example of sand and glue watered down to a paint-like consistency.
If you want to add a thick layer of cob to your wall to allow for the sculpting of niches or other high-relief work, I would recommend making your foundation that much wider and building up the cob from the bottom. It would also help to leave some sort of pins (framing nails, for example) sticking out from the mortar joints to improve the connection between the Rastra wall and the cob. Applying the cob on the interior rather than the exterior surface makes a lot more sense from both a thermal and a weather-protection point of view. Let us all know what works!
Q: I am wondering if it is possible to cob over interior concrete block walls. This is an existing house with unfinished concrete block walls we would like to use natural plastering and cob to create the natural earthen made look. Is there anything special that we need to apply to the concrete first.
A: Yes, I'm sure that what you're proposing is possible. The trick will be finding a good way to attach the cob to the concrete. If you are doing only a fairly thin earthen plaster, you can first apply an "adhesion coat" made of flour paste or white glue and coarse sand. When it dries, that will give the wall enough "tooth" or texture to hold onto the plaster.
If you want to apply cob more than an inch or so thick (which might be nice to mask the harsh square lines of the concrete block and introduce some sculptural elements) you will need to create some sort of an armature to support it. I would use some sort of concrete bolt or screw (many types are available at your local building supply store). Drive them in until firmly anchored but leave them sticking out a few inches. Then you can wrap galvanized fencing wire around the heads to make a sort of skeleton for your cob elements. The length and spacing of the bolts depends on how much weight of cob you will be attaching, and on whether the cob is supported on the floor below or whether it is free-floating attached only to the concrete. But in general I would recommend something like a six inch spacing both vertically and horizontally.
Q: I am planning to build a cob home on the beach and am curious how practical it would be to plaster my shower wall with an earthen plaster and imbed seashells over the entire wall. If I do this, what would be the best way to seal it?
Q: What about using cob for the walls of a double-chambered compost toilet and also for graywater beds indoors and outdoors and sinks? What is the best way to waterproof these?
A: Earthen materials like cob do not tend to be durable when subjected to lots of moisture. I wouldn't recommend using cob in direct contact with the ground, except in the very driest of climates. I have heard of people making cob sinks and bathtubs and sealing them with linseed oil. I imagine that if you re-oiled them frequently they would last for a while, but I don't know how long.
Q: Where would I purchase earthen plaster? If we took all the dry wall and insulation out of the wall could we put straw where the insulation went, then chicken wire, then mix straw into the earthen plaster and place on top of that?
A: There is a company now manufacturing earthen plaster and selling it dry mixed in bags - just add water and apply. The company is called American Clay (www.americanclay.com) and I have no personal experience with their products. Most people mix up their own earthen plaster from clay that is either found "in the wild" or purchased dry in bags, with sand and fine fiber (such as chopped straw) added. A great resource for earthen plasters is "The Natural Plaster Book" by Cedar Rose Guelberth and Dan Chiras. I would not recommend applying it over chicken wire - metal and earth are very dissimilar materials that don't bond easily. Also the chicken wire will probably not give you the stiffness you need for easy application of plaster. A better choice for the situation you're describing would be reed mat, which is available at hardware stores for landscaping purposes. Another option would be bamboo, small saplings or thin milled wooden lath, running horizontally and stapled or nailed to your studs.
Q: I live in the Jordan valley, 200 meters below sea level. What is better to use for building: adobe or cob?
A: Cob and adobe have very similar structural properties, as the components used to make them are practically identical. The main difference structurally is that well-built cob, because it lacks all the "cold joints" or seams intrinsic to adobe, should fare substantially better during an earthquake. Other factors contributing to your choice of one technique over the other include weather, availability of skilled labor, and your personal aesthetic. It is possible to build with cob in cool, damp weather unsuitable to adobe, although I don't imagine that being an issue where you are. It may be that there are local builders trained in the use of adobe but not in cob. If so, it might be simpler to go along with the local vernacular rather than having to train a work force in a new technique. Of course, if you plan to do most of the building yourself, that also becomes less of a concern. Cob, because it uses no forms, is very easy to sculpt into rounded and organic shapes. If you like that look, cob may be for you. Or you could use a combination of both techniques, taking advantage of the speed of erecting adobe walls (assuming you can buy pre-made adobe blocks) and the sculptural properties of cob within the same building.
Q: I am currently developing plans for a cob house outside of Waco, Texas. Since the winters are mild and relatively dry I am considering building with a full berm on the north side and with only partial berms on the east and west sides (straw bale for the north and west walls, earthen floors, interior partitions, and thick plaster). Does this plan make sense, or is there a better way to build a cob structure in this climate?
A: Not to say that it couldn't be done, but I don't know of anyone who has successfully built a bermed cob structure. Trapped moisture inside the wall is death to cob, so you have to be extremely careful about drainage and waterproofing. The same goes for straw bale. I would generally not recommend using either material below grade. The exception could be in a desert climate with extremely well-drained soils, where there is no chance at all of moisture infiltration from wet soil into the below-grade part of the building. A safer option would be, if you want to berm, to use a material (such as concrete or stone) that will not be damaged if it gets wet up to and a little bit past grade, and then switch to cob or straw only above that level.
If the winters are so mild, I question the value of berming the building at all. I imagine that your intention is to keep it cool in the summer. However, a very high-mass wall material such as cob, in combination with earthen or other masonry floors, will achieve the same effect with less potential complication.
Q: I have obtained funds for an urban sustainable garden from my local council. However our ambitious plans for curved raised beds have been turned down on grounds of cost. Is it possible to make the walls for the raised beds out of Cob? Would it need to be treated in any way to prevent moisture damage? Would cob be strong enough to resist vandalism?
A: (Kelly) Cob would not be a very good choice for raised garden beds because it is basically mud reinforced with straw and allowed to dry; when it gets damp it will return to mud. Even with a plaster of lime cob is best protected by a roof to keep it dry.
Perhaps a better choice that should be quite inexpensive would be the use of earthbags for this project. Either the individual polypropylene bags, or continuous polypropylene tubing can be filled with earth and formed into lovely curving shapes for garden beds. Two strands of barbed wire are usually placed between the courses to help keep them in place. The bags need to be plastered to protect them from the UV in the sun, but they are unaffected by moisture, and even if the soil inside gets damp, they will not generally deform. Either a lime plaster or cement stucco is rather durable, but of course nothing can resist all sorts of vandalism.
Q: I plan to build a 5/8 geodesic dome house using straw bales and cob on the exterior. My biggest worry, as you can imagine, is waterproofing, especially as I live in a fairly rainy climate. Do you think that earth plaster and linseed oil would do the job? Do you have a formula for such a plaster? How many coats should I use and how often should I renew the protective layer?
A: I'd like to do whatever I can to dissuade you from your plans to build a cob and straw bale dome. I think that is a very dangerous undertaking, especially in a rainy climate. All building materials expand and contract with changes in temperature and humidity. I don't believe there's a plaster in existence that would keep your dome leak-proof for more than a few years. Once the plaster cracks and water gets through, the cob and especially the bales are capable of absorbing enormous amounts of water. Both will become very heavy and at the same time lose their rigidity and compressive strength. This is a disaster waiting to happen. Ken Kern, the pioneer of owner-building in the 60's and 70's, died in an experimental earthen dome that he built and plastered with concrete stucco. He was sleeping in it the night of a huge rainstorm, when enough water finally got through the cracks in the plaster and into the cob walls to snap the concrete structural reinforcements. Please learn from Ken's death and don't put yourself in a similarly dangerous situation.
You may be saying, yeah, I know, but I will do the necessary maintenance and replace the plaster every few years, or every year if I have to. That may be so (although it sounds like a set-up for a huge amount of ongoing work for the rest of your life), but can you be sure that whoever else might be living in the house after you're gone would understand the danger and keep up on the maintenance? Are you comfortable gambling with the lives of all the building's future inhabitants?
So I would say that plaster as a waterproofing surface is out. The next possibility would be to use a flexible waterproof membrane such as EPDM to protect the outside of your dome from moisture. Unfortunately, you would thereby encase your earth and straw building in a material with no vapor permeability, opening yourself up to a whole other set of moisture problems resulting from condensation between the earth/straw and the membrane. Toss that idea out the window.
The final option would be to build your dome, then build a waterproof roof structure over it. This is the most feasible idea, but still not really a good one. First of all, it's a big duplication of effort, since one of the main points of the dome in the first place is to avoid having to build a separate roof structure. And secondly, if your roof were ever to leak for any reason (which eventually happens with most roofs) you would be right back in the first scenario, with potentially waterlogged earth or straw over your head, in the most dangerous possible location.
Incidentally, I'm not alone in my distrust of domes as a building system. LLoyd Kahn, who published the 70's classics "Shelter," "Domebook," and "Domebook 2" took the latter 2 books out of print after coming to the conclusion that geodesic domes are impractical. If you look around the world at vernacular building traditions, you will only find earthen domes in extremely dry climates. There's good reason for that.
Q: I'm thinking of building a cob sculpture of a woman. I live in Ireland and am wondering about the weather here. If she was about 10ft high how long would she take to dry, and would a lime plaster be sufficient to save her from the rain? I was thinking of a hazel structure underneath. Is there anything I need to be aware of / watch out for? Do you know of anyone who makes sculptures from cob? How long would it last in rainy old Ireland, and could you point me in the direction of some good books?
Your cob sculpture project sounds exciting! It's very difficult to predict how long cob will take to dry. If you have high temperatures, low humidity, and wind, your sculpture could be dry in a week or two. If the weather is cool and/or damp, drying will be much slower. I've seen wet cob sit for months during a rainy Oregon winter without getting dry.
Q: I am living at a sustainable demonstration house at the University of Montana in Missoula. We are rehabilitating a large garage building on this site to become a meeting space and community center. I traveled through Denmark this summer visiting ecovillages and saw quite a few beautiful cob buildings, so I am particularly interested in putting in a cob floor and potentially some cob interior walls in this garage. The first issue for us is the existing concrete floor. I read that you thought a cob overlay of a concrete wall would work without too much problem, do you think the same would be true of building our floor on top of concrete? Any moisture issues we might want to consider?
A: When installing an earthen floor over concrete, moisture is going to be your major concern. If the existing slab is not well drained, moisture can work its way through the concrete and get into the earth, causing it to get soft and/or grow mold. Sealing the slab first with some sort of concrete sealer might be advisable, and improving the perimeter drainage if it seems inadequate.
Comment: Here's a picture of a Truro oven that we recently completed here in Truro, Nova Scotia. Just thought you might like to see it. It turned out beautifully!
Q: I would like to construct a cob hearth in my house for my wood stove to sit on. The vertical portion of the hearth will also be acting as a bathroom wall - which I thought would be a nice way to get the heat from the wood stove to transfer to the inside of the bathroom. I live in NH. It is already October and I am already using the wood stove. I could turn the stove off for a week to do the construction but then I am concerned that running the stove so close to new cob will cause the cob to dry too quickly and crack. Should I just scrap the project until spring? The stove is now sitting on durarock...ugly but functional. :)
A: It is possible that the cob could crack if you get the wood stove going really hot before it is dry. It is also possible that the cob could take a really long time to dry, especially on the bathroom side, and that you could get mold growing on it. Cob drying time is affected not only by temperature but also by air movement and humidity. When people are doing indoor cob projects I generally recommend leaving all of the windows open and putting fans in the windows to get the moisture out of the house. You probably won't want to do that in winter in New Hampshire. So for all of those reasons it would be prudent to wait until the weather is warm.
Q: I like cob for mortaring cordwood. What material/proportion do you advise?
A: Yes, in my experience a cob-like mixture works well as mortar for cordwood masonry. I would recommend a mixture of clay soil, sand, and either chopped straw or horse manure. The fine fiber will make the mortar easier to point (make smooth) as you go, without long messy straws hanging out that you would need to trim later. The exact proportions depend on your clay soil. You could start with a mix that is 2 parts clay soil, 2 parts sand, and 1 part fiber. If it cracks as it dries, add more sand. If it seems soft and crumbly, add more clay.
The only place where a cob-like mortar will not work well is where it gets wet frequently. So at the bottom of the wall, if you don't have a high foundation to elevate the cordwood masonry wall above the level of rain splashing off your roof, you might consider using a mixture with cement or lime as the binder.
Q: To me the most intriguing dwelling shape is the tipi. Would a tipi shaped cob structure be possible or maybe a truncated cone with a flat roof.
A: It's theoretically possible to build a tipi-shaped cob structure. However it would be somewhat of an engineering challenge. Cob walls are very heavy. If they are vertical, they support themselves, as they are also very strong. But at an angle like a tipi wall, you would need a very strong tension structure to keep all that weight from falling over, and possibly also an even more massive foundation as a counterweight. My recommendation would be to consider a vault or dome shape, which are compressive structures well suited to earthen construction. Or if you want your tipi shape then to go with a lighter wall system like wattle-and-daub. From a practical perspective, I would find a tipi-shaped home difficult to inhabit efficiently. There would be a lot of wasted space around the perimeter that isn't high enough either to stand upright in or to receive furniture. And if you live anywhere where earthquakes are possible, it would be very dangerous to have so much earthen mass overhead. This is a problem with vaults and domes as well as a tipi-shape, and explains why these forms are typically found only in seismically stable areas.
(Kelly) Michael Smith suggested that I send you a link to some pictures of the earthbag house that I built in Colorado because it has somewhat of a tipi feeling inside. There is a description of this house at earthbagbuilding.com
Something else I might add to Michael's comments about building a cone (or vault or dome for that matter) with cob is the danger of failure if the cob gets saturated with moisture from rain. Even with an impervious layer of plaster or paint this is a danger.
Q: I have been looking into building a cob house and want to know if it is possible to make a cob house with an earthen roof and then push dirt around the structure to make it sort of a hill? I am concerned about moisture build up. I want to be able to grow wild grasses native to Oklahoma. I hate to sound silly but the closest thing I can find to what I am speaking of is a Hobbit House. I am concerned with the weight pushing on the walls.
A: Covering a cob house in earth doesn't sound like a good idea to me for 2 reasons. The first is structural; earth piled up against a wall can exert an enormous amount of sideways pressure on the wall. A cob wall might or might not be able to withstand that pressure; it would depend on the qualities of the site soil, the strength of the cob, and the height, thickness and shape of the wall. But calculating ahead of time whether or not the wall would be strong enough would be hard to do since so little engineering testing has been done on cob.
Secondly, it's important for the longevity of a cob building that the walls be able to "breathe." That means that water vapor needs to be able to find its way out of the wall to prevent moisture building up inside the wall. If too much moisture builds up in the cob, the wall can lose its strength and collapse. If you were to pile earth up against the outside of a cob wall, you would have to seal the outside of the cob wall to prevent moisture wicking from the damp earth into the wall. But whatever you used to seal the wall would also prevent it from breathing. So I don't think this is a good strategy except maybe in an extremely dry place where it almost never rains.
If you're attached to building a "hobbit house," you could do so using concrete as your wall material. There are a couple of books out there that will tell you how, including "Earth-Sheltered Houses" by Rob Roy. But if you want to build your walls out of cob, stick to a living roof but let the walls breathe.
Q: I have recently been given an old mobile home, which is water tight but cold and tinny. I would love to cover it in cob, rather than rip it all down to start again. Would it be possible to cover it in cob? I've seen people apply cob to walls covered in chicken wire....would it be a similar solution?
A: Cob usually refers to a structural earthen wall made out of clay soil, sand and straw. You can take the same (or similar) ingredients and apply them in a non-structural way to an existing wall surface. I would refer to that as an earthen plaster. You can build the earthen plaster out as thick as you wish to provide thermal mass and/or sculptural texture to a flat, industrial surface. You would probably want to attach some sort of lath to the existing wall before applying the earthen plaster (unless the plaster coat is very thin, in which case you could simply paint on a mixture of sand and glue to provide "tooth.") There are several things you can use for lath; my favorite is reed mat which you can buy in rolls at the hardware store. Run the reeds horizontally and secure the mat to the wall with lots of screws through the wire lashings. Chicken wire could also work as lath if you stretch it tight, but is less pleasant to work with.
I'm presuming you want to plaster the inside of your mobile home with earth. That could increase temperature stability, reduce mold problems due to condensation, and improve the look and feel of the interior. Just to be sure you understand what you will NOT achieve this way: you will not get a breathable wall and you will not significantly improve the insulation. A mobile home will always be somewhat cold and tinny no matter what you do to it; it's in the nature of the materials the structure is made of.
I do not recommend applying earthen plaster to the exterior of the mobile home. Although it is technically possible, it would be a large project and will leave you with ongoing maintenance, in exchange for very little improvement to the functionality of the building.
Q: My partner and I are thinking about building a solar and wind powered cob sauna and steam room for my parents. What would you recommend for the interior of a steam room that could be in constant use? Would sealed tiling affect the breathability of the structure and encourage damp? I understand it will probably have to be a hypothetical answer but how long do you think the traditional lime plaster would last in a typical 40 degrees Celsius humidity?
A: It so happens that I've built a cob sauna and plastered the interior with lime/sand. After 13 years of weekly use there is no sign of plaster degradation or moisture problems, so that's what I'd recommend for you. Sealed tiles would also probably work, as long as the exterior surface of the cob walls are left breathable. Important details to consider are going to be the floor shape and material (you don't want puddled water to soak into the cob walls) and the ceiling, catching and diverting all the condensation before it gets into the walls from above.
Q: I am in the planning stage of building my container home in South Florida. The idea of living in a metal box kinda wigs me out so I wanted to use cob for my interior walls to soften it up. Do you think that would create any mold problems if I were to use denim insulation?
A: I'm not sure what exactly you have in mind; whether you are talking about freestanding cob for interior partitions or an earthen plaster surface over your insulated exterior walls. Either one sounds feasible, but keep in mind that no matter what you do on the inside, you will still be living in a metal box. Metal does not breathe and so I would expect lots of condensation, which can lead to mold. It might be possible to mitigate this with a high-tech air-exchange system, but I don't know of any way to address the potential moisture and mold issues using passive techniques and natural materials.
Q: I am thinking of building a cob home underground, best described as a hobbit house. How would one go about keeping the cob from damage after it's buried?
A: I would not recommend building with cob below grade. To last a long time without moisture problems, it's essential that cob walls be able to release any water they have absorbed, ideally to both sides (inside and out). That is obviously not possible if you waterproof the exterior of the walls to prevent them from becoming soaked by absorbing moisture from the soil. If you are in an extremely dry location (i.e. the desert), you might be able to get away with the cob releasing moisture only to the interior of the building. Otherwise I would suggest you find another building system, such as gravel-filled bags.